The Direct Reactor Auxiliary Cooling System (DRACS) is a passive heat removal system proposed for Fluoride Salt-Cooled High-Temperature Reactors (FHRs). It features three coupled natural circulation/convection loops completely relying on buoyancy as the driving force. The objective of this research is to test and model the heat transfer performance of a DRACS low-temperature design. To achieve this, based on the scaling analysis, a scaled-down low-temperature DRACS facility was designed and is being constructed. This low-temperature design will be used to acquire data to examine the coupling within the subsystems and to benchmark a preliminary computer code developed for the DRACS design.
There are four main components to the Low-Temperature DRACS facility. In the core, three cartridge heaters simulate nuclear fuel and will provide the decay heat for the system. For this test the nominal power will range from 2-50 kW. Connecting the primary loop and the secondary loop is the DRACS Heat Exchanger (DHX), a shell and tube heat exchanger. In the DHX, coolant energy from the primary loop is transferred to the secondary loop. The Natural Draft Heat Exchanger (NDHX), a tube-fin heat exchanger, couples the secondary loop to the atmosphere. In the NDHX, energy is transferred from the secondary coolant to the atmosphere. The primary loop also contains a fluidic diode. For the low-temperature design, a simulated sub-system made of automatic ball valves and globe valves are used for the fluidic diode. This allows the resistance to be controlled for natural and forced circulation flows.
The coolant chosen for both the primary loop and secondary loop is water. The primary loop operates at a pressure of 10 bars with a maximum average temperature of 77oC. The secondary loop operates at atmospheric pressure and has a maximum operating average temperature of 65oC. To monitor the coolant temperatures ten thermocouples are placed in key areas within the loops. In addition pressure transducers and ultrasonic flow meters are used to measure the system's pressure differentials and flow rates.
To simulate the Low-Temperature DRACS Test Facility two MATLAB codes are created solving the energy balance equations and the integral momentum equations within the DRACS facility. Based off of the MATLAB code developed for the prototypic model, the first code models the DRACS behavior after the pump trip and flow reversal. This code is also a mesh dependent code which allows flexibility in computing time and result accuracy. The second code models the behavior of the system with the pump trip by studying the flow behavior of the three branches within the primary loop before and after the pump trip.
The DRACS designed and constructed in this study will be used to collect data on the three coupled loops. A comparative analysis will be performed between the experimental data and the outputs obtained by the code. Knowledge from this analysis will be used to improve input parameters in the code as well as any needed improvements toward the low-temperature design. With a firm understanding of the low-temperature DRACS design, a basic understanding will be laid for future work, such as the scaled- down Fluoride Salt-Cooled High-Temperature DRACS design.